In general, display of a liquid crystal display is realized by applying an electric field to liquid crystal molecules in a liquid crystal layer sandwiched between a pair of substrates to change the alignment direction of the liquid crystal molecules and utilizing the resulting change in the optical property of the liquid crystal layer. Conventionally, a liquid crystal display of a so-called active drive type having a switching device such as a thin-film transistor for each pixel is represented by a TN (Twisted Nematic) display scheme in which an electrode is provided for each of a pair of substrates sandwiching a liquid crystal layer between them, the direction of an electric field applied to the liquid crystal layer is set to be substantially perpendicular to the interface between the substrates, and the optical rotatory power of the liquid crystal molecules constituting the liquid crystal layer is utilized to achieve display. In the liquid crystal display of the TN scheme, a small viewing angle is regarded as the greatest problem.
On the other hand, “Patent Document 1,” “Patent Document 2,” “Patent Document 3,” “Patent Document 4,” “Patent Document 5” and the like have disclosed an IPS scheme in which an inter-digital electrode formed on one of a pair of substrates is used to produce an electric field having a component substantially in parallel with the substrate surface to rotate liquid crystal molecules constituting a liquid crystal layer in a plane substantially in parallel with the substrate and the birefringence of the liquid crystal layer is used to realize display. The IPS scheme has advantages such as a wider viewing angle and a lower load capacity due to the in-plane switching of the liquid crystal molecules as compared with the conventional TN scheme. The IPS scheme is considered as a new and promising liquid crystal display which will replace the TN scheme and has made rapid advances in recent years. In addition, another type of the IPS scheme has been disclosed in “Patent Document 6” in which at least one of paired electrodes for applying an electric field to a liquid crystal layer is made of a transparent conductive film to improve transmittance.
The liquid crystal display of the IPS scheme (abbreviated as IPS-TFT-LCD) with favorable viewing angle characteristics (luminance contrast ratio, tone and color reversal) and bright display represents prospective technology for monitors or televisions with a large display area. In the liquid crystal display, an alignment control film provided with a liquid crystal alignment control ability is formed on the interface between a liquid crystal layer and each of a pair of substrates sandwiching the liquid crystal layer between them. However, to put IPS-TFT-LCDs for supporting large screens of 20 inches or more into practice use in the future, it is necessary to develop a new structure and process for large-size displays (large panels).
In particular, for an IPS-TFT-LCD having many stepped structures on a surface opposite to a liquid crystal layer, it is difficult to perform uniform alignment processing on an alignment control film over a large screen. A margin in performing the alignment processing on the alignment control film is significantly smaller than that of the conventional TN scheme, especially a normally open type TN scheme which is predominant at present (bright display at low voltage and dark display at high voltage). The reasons for the small margin include three points described below as (1) to (3).
(1) Stepped Structure
In the IPS-TFT-LCD, it is necessary to provide a number of elongated electrodes (which may be referred to as inter digital electrodes) having a width of approximately several microns in principle. This causes minute stepped structures to be formed therein. The height of the step depends on the thickness of the electrodes or the shapes of various films formed thereon, and typically is equal to 0.1 micron (mμ) or larger. An alignment control film (also referred to as an alignment film) made of a polymer film such as polyimide is formed in the uppermost layer of those films.
In conventional mass production technology, the alignment control film is subjected to rubbing processing to provide a liquid crystal alignment ability (initial alignment). Meanwhile, a cloth for the rubbing is formed by binding thin fibers with a thickness of approximately 10 to 30 microns. Essentially, each of the thin fibers provides shearing force in a predetermined direction for a local portion of the alignment film to perform the processing of giving the liquid crystal alignment ability. While very thin fibers of approximately several microns are present as the fibers, such very thin fibers have not been put into practical use since rigidity for providing certain frictional force is required for the rubbing. The interval between the electrodes in the IPS scheme is approximately 10 to 30 microns which is substantially the same as the diameter of the fibers, so that sufficient rubbing is not performed near the steps and misalignment tends to occur. The misalignment leads to reduced image quality such as a higher black level, an associated lower contrast ratio, and uneven luminance.
(2) Alignment Angle
In the IPS-TFT-LCD, the initial alignment direction needs to be set in principle at a certain angle or more shifted from the direction in which the electrode extends or the direction perpendicular thereto. The electrode refers to a signal wiring electrode, a common electrode in pixels, and a pixel electrode. The definition of the initial alignment direction through the rubbing requires the fibers of approximately 10 to 30 microns to rub in a predetermined angular direction as described above. However, the step of the wire such as the signal wiring electrode, the common electrode in pixels, or the pixel electrode extending in a certain direction at their ends draws the fibers toward the step from the set angle to produce misalignment, thereby reducing image quality such as a higher black level.
(3) Expression of Dark Level
One of the characteristics of the IPS-TFT-LCD is excellent expression of a dark level (black display). Thus, misalignment is easily noticeable as compared with the other schemes. In the conventionally normally open type TN scheme, the dark level is provided while a high voltage is applied. In this case, most of liquid crystal molecules align in the direction of the electric field which is one direction perpendicular to the substrate surface at a high voltage, and the dark level is provided from the relationship between the arrangement of the liquid crystal molecules and the placement of a polarizing plate. Thus, the uniformity of the dark level hardly depends on the initial alignment state at a low voltage in principle. In addition, since human eyes recognize uneven luminance as a relative ratio of luminance and make response close to a logarithmic scale, they are sensitive to variations in the dark level. From this viewpoint, the conventional normally open type TN scheme in which the liquid crystal molecules are forcedly arranged in one direction at a high voltage is advantageous in that it is not sensitive to the initial alignment state.
On the other hand, in the IPS scheme, display of a dark level is performed at a low voltage or no voltage, so that it is sensitive to disturbance of the initial alignment state. In particular, when homogeneous alignment is used in which the alignment directions of liquid crystal molecules are in parallel with each other on an upper substrate and a lower substrate, and the light transmission axis of one of polarizing plates is set in parallel with the alignment direction of the liquid crystal molecules and the light transmission axis of other polarizing plate is set orthogonally thereto (called a birefringence mode), polarized light incident on the liquid crystal layer is transmitted with almost no disturbance of lineal polarization. This is effective in providing excellent expression of the dark level.
The transmittance T in the birefringence mode is expressed by the following equation:T=T0·sin2{2θ(E)}·sin2{(π·deff·Δn)/λ}where T0 represents a coefficient which is a numerical value determined mainly by the transmittance of the polarizing plate for use in the liquid crystal panel, θ(E) represents an angle between the alignment direction of liquid molecules (the effective optical axis of the liquid crystal layer) and the polarized light transmission axis, E represents an applied electric field intensity, deff represent the effective thickness of the liquid crystal layer, Δn represents the refractive index anisotropy of liquid crystal, and λ represents the wavelength of light. The product of the effective thickness deff of the liquid crystal layer and the refractive index anisotropy Δn of the liquid crystal, that is, deff·Δn is called retardation. The thickness deff of the liquid crystal layer does not refer to the thickness of the whole liquid crystal layer but corresponds to the thickness of the liquid crystal layer which actually changes in the alignment direction when a voltage is applied thereto. This is because the liquid crystal molecules near the interface of the liquid crystal layer do not change in the alignment direction due to the influence of anchoring at the interface even when a voltage is applied thereto. Thus, assuming the thickness of the whole liquid crystal layer sandwiched between the substrates is dLC, the relationship deff<dLC is always found between the thicknesses dLC and deff. The difference between them can be estimated at approximately 20 nm to 40 nm, although it depends on the liquid crystal material used in the liquid crystal panel and the type of the interface in contact with the liquid crystal layer, for example the material of the alignment film.
As apparent from the above equation, the term sin2{2θ(E)} depends on the electric field intensity, and the luminance can be adjusted by changing the angle θ in accordance with the electric field intensity E. For the normally close type, polarizing plates are set to satisfy θ=0 when no voltage is applied, and it is sensitive to disturbance of the initial alignment direction.
In this manner, the uniformity of alignment is a very important factor in the IPS scheme, and problems in the currently used rubbing technique have become apparent. In general, the rubbing alignment processing includes many problems associated with the rubbing processing technique such as TFT breakage due to static electricity produced by friction, unfavorable display due to misalignment from disordered fiber ends of a rubbing cloth or dust, and the need for frequent exchanges of rubbing cloths. For the purpose of solving the problems associated with the rubbing alignment processing, a so-called “rubbing-less” alignment technique for aligning liquid crystal molecules without the rubbing has been studied and various processes thereof have been proposed. Among other things, a process has been proposed in which polarized ultraviolet rays or the like are irradiated to the surface of a polymer film to align liquid crystal molecules without the rubbing.
As an example, a process disclosed in “Non-Patent Document 1” is characterized in that it does not require the conventional rubbing processing and realizes the alignment of liquid crystal molecules in a predetermined direction through irradiation of polarized light. The process is advantageous in presenting no problems such as damages on the film surface and static electricity associated with the rubbing technique and providing a simpler production process in view of industrial production. The process has attracted attention as a new liquid crystal alignment processing process without using the rubbing processing.
As a material of the liquid crystal alignment film used in the previous reports, the use of a polymer compound having a photoreactive group in the side chain of a polymer has been proposed for the need to provide photochemical sensitivity to polarized light. A representative example thereof is polyvinylcinnamate, in which case it is thought that dimerization in the side chain through light irradiation develops anisotropy in a polymer film to align the liquid crystal. Another proposal involves dispersing low-molecular dichroic azo dye in a polymer material and irradiating a film surface with polarized light to allow the alignment of liquid crystal molecules in a predetermined direction. In addition, the alignment of liquid crystal molecules achieved by irradiating a particular polyimide film with polarized ultraviolet rays or the like has been reported. In this case, it is contemplated that the light irradiation decomposes the polyimide main chain in a certain direction to develop the liquid crystal alignment.                Patent Document 1: JP-B-63-21907        Patent Document 2: U.S. Pat. No. 4,345,249        Patent Document 3: WO91/10936        Patent Document 4: JP-A-6-22739        Patent Document 5: JP-A-6-160878        Patent Document 6: JP-A-9-73101        Patent Document 7: Japanese Patent No. 3303766        Patent Document 8: JP-A-11-218765        Non-Patent Document 1: W. M. Gibbons et al., Nature, 351, 49 (1991)        